In recent times it has become increasingly important to determine the location of a user using a mobile device in order to provide appropriate services to users, such as restaurant or shop recommendation or to locate the user in order to provide emergency medical services. On the other hand it has also become important to locate a user to provide appropriate law enforcement regarding criminal subjects.
For determining the location of a mobile device commonly satellite supported methods have been established, such as using the Global Positioning System which is widely used to determine the position of cars and used for route planning and navigation. Such methods however have the disadvantage, that they require special receivers and transmitters which are only suitable for navigational purposes and require an additional technical effort as well in the user device and in terms of infrastructure to supply the corresponding satellites emitting proper navigation signals.
Due to the high competition in the mobile infrastructure and devices market there is a strong tendency among the competitors to keep mobile devices and their infrastructure technically simple, in order to remain cost-competitive on the market or to gain a competitive advantage. Therefore, there is a strong need to provide location services for mobile devices which are technically simple while at the same time being reliable and efficient as well as sufficiently accurate to determine the position of a user in order to be able to provide to him or her suitable services.
Accordingly methods have been established to determine the position of a user, by receiving a signal emitted by the user's mobile device at fixed receiver stations, and based on the different signal propagation from the user device to the different fixed receivers to calculate a position of the user device. In such an environment it is crucial that all the devices involved in the location determination are basing their corresponding analysis on a common clock. The positional accuracy of such a system is directly related to the accuracy of the clocks and the corresponding synchronicity of the clocks which are used to determine the propagation delays involved in the location determination.
One such method is based on a time difference of arrival TDOA and uses the time difference it takes for a signal to travel to two destinations as an indirect method of calculating a distance. With a minimum of three base stations, for instance, receiving the signal from a handset, the difference in time it takes for the signal to reach each tower of a base station can be used to triangulate the position of the mobile unit. TDOA systems do not need any specialized antennas and as such the infrastructure is kept simple. When a mobile that has to be located transmits, the arrival time of the target mobile signal is recorded by a TDOA location measuring unit at each base station or access point which is able to receive the signal. Since the mobile's signal travels at a constant speed (the speed of light), comparison of the arrival time of the signal for any two sides allows a straightforward calculation to determine the mobile's relative position to each side. When plotted, this relationship describes an imaginary hyperbola in space. The target mobile is located somewhere on this curve although additional information is required to determine precisely where. When the same calculation is made involving measurements from a third base station or access point side, calculating a difference of arrival time from sites A, B, C, e.g. between either sites A and C or between sites B and C, an independent positional hyperbola can be described. The point at which the two hyperbolas AB and BC intersect is the location of the target mobile. Commonly, TDOA requires accurate time synchronization at the base station or access points, but not necessarily at the target mobile device. It is immediately apparent, that inaccuracies in the clock measurements may lead to large location errors. A high clock accuracy and the corresponding master clock and an associated synchronization procedure require however technically complicated solutions which on the other hand require adaptations at the infrastructure devices such as base stations and for instance access points in order to provide them with a suitable clock reference.